How to use Interlocked.CompareExchange as an inter-process lock?

Question

I'm obviously doing something wrong here. I always get different number running this code:

var counter = new ConcurrentCounter();

var count = counter.Count();

Console.WriteLine(count);

public class ConcurrentCounter
{
    private int _id;
    private int _lock;
    private int _counter;

    public int Count()
    {
        var threads = Enumerable.Range(0, 10).Select(i => new Thread(Worker)).ToArray();

        foreach (var thread in threads) thread.Start();

        foreach (var thread in threads) thread.Join();

        return _counter;
    }

    private void Worker()
    {
        for (var i = 0; i < 1000000; i++)
        {
            while (true)
            {
                var id = Interlocked.Increment(ref _id);

                var current = Volatile.Read(ref _lock);

                //Begin a locked block
                if (Interlocked.CompareExchange(ref _lock, id, current) == current)
                {
                    var value = Volatile.Read(ref _counter);

                    Volatile.Write(ref _counter, value + 1);

                    //Release the lock
                    Volatile.Write(ref _lock, current);

                    break;
                }
            }
        }
    }
}

What is wrong here? How can I get a working lock using Interlocked.CompareExchange? I cannot use usual C# lock as it will be used for inter-process shared memory lock.

Answer 1

The problem with your code is that every time a CAS fails, you will retrieve the _lock value again, so the next CAS will definitely succeed, leading to this useless comparison. You need to block the thread until you've released the lock.

This is actually a simple spin lock, you do not need additional _id to implement the lock, you only need to define two values:

int lockValue = 1;
int freeValue = 0;

for (var i = 0; i < 1000000; i++)
{
    //Begin a locked block
    while (Interlocked.CompareExchange(ref _lock, lockValue, freeValue) != freeValue)
        Thread.Sleep(0);

    var value = Volatile.Read(ref _counter);

    Volatile.Write(ref _counter, value + 1);

    //Release the lock
    Volatile.Write(ref _lock, freeValue);
}